SCIENCE CHINA Information Sciences, Volume 61, Issue 10: 100305(2018) https://doi.org/10.1007/s11432-018-9509-y

## Gray codes over certain run-length sequences for local rank modulation$^\dagger$

• AcceptedJun 28, 2018
• PublishedAug 21, 2018
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### Abstract

In the local rank modulation (LRM) scheme, a sliding window produces a sequence of permutations by moving over a sequence of variables. LRM has been presented as a method of storing data in flash memory, which represents a natural generalization of the classical rank modulation scheme. In this paper, we present a study on Gray codes over certain run-length sequences for the $(1,2,n)$-LRM scheme to simulate virtual multilevel flash memory cells while maintaining the advantages of LRM. Unlike previous studies on the LRM scheme, we present Gray codes over certain run-length sequences in the $(1,2,n)$-LRM scheme. This class of Gray codes can overcome the drawback of the many distinct charge levels required in the rank modulation scheme and in certain Gray codes for LRM. Furthermore, we demonstrate that the proposed codes have an asymptotically optimal rate.

### Acknowledgment

This work was supported by National Basic Research Program of China (973) (Grant No. 2013CB834204) and National Natural Science Foundation of China (Grant No. 61571243).

### References

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• Table 1   Parameters of $G,G_1,$ and $G_2$
 Gray code $n$ $N$ $\Psi$($\cdot$) Parameter constraint $G_1$ [12] $m^2$ $\mathrm{lcm}({\binom{m}{w}}^{m-2},m)\cdot(w+2)\cdot~m$ $\geq~m$ $m\geq~w,~w~~\text{is~a~positive~integer}$ $G_2$ [11] $m^2$ $O(2^{(m-1)(m-3)})$ $\geq~m$ $m\geq~4$ $G$ $m^2$ $O(2^{m^2\cdot~{\log_2\alpha}_d})$ $d$ $d=2d_1+3,~d_1\geq~3,~m\geq~2d_1+4,~~\alpha_d~~~\text{is~defined~in~Lemma~2}$
•
 Algorithm $1$ Set the initial cell charge levels $\boldsymbol{c}_{(0,0)}=(c_{(0,0)}^1,c_{(0,0)}^2,\ldots,c_{(0,0)}^n)$ such that$\boldsymbol{f}_{{\boldsymbol{c}_{(0,0)}}}=\boldsymbol{g}_{0}^{0}$ and $\boldsymbol{c}_{(0,0)}\in~$ $(0,\frac{d+1}{2}]^{n}$ Input: current cell configuration $0^n$, the initial value $\boldsymbol{g}_{0}=\boldsymbol{v}_{s_{m-1}}\boldsymbol{v}_{s_{m-2}}\cdot\cdot\cdot\boldsymbol{v}_{s_1}\underline{\boldsymbol{v}_{s_{0}}}$. Output: new cell configuration $\boldsymbol{c}_{(0,0)}=(c_{(0,0)}^1,c_{(0,0)}^2,\ldots,c_{(0,0)}^n)$. $k\Leftarrow~0,~T\Leftarrow\emptyset,~i\Leftarrow~3,~s\Leftarrow~0,~t\Leftarrow~0,j~\Leftarrow~0$ repeat $c_{(0,0)}^{kn_1+2}\Leftarrow~1$ (let the lowest charge level be $1$). while $(3\leq~i\leq~n_1)\{$ case 1: $(s=0~\text{and}~\boldsymbol{g}_{0}^{0}(kn_1+i)=0)$ $c_{(0,0)}^{kn_1+i}\Leftarrow~c_{(0,0)}^{kn_1+i-1}+1$ ($\delta=1$ is the minimum charge difference required to distinguish two distinct charge levels and this cell corresponds the bit $0$). case 2: $(s=0~\text{and}~\boldsymbol{g}_{0}^{0}(kn_1+i)=1)$ $s\Leftarrow~i,T\Leftarrow~T\cup\{i\}$ ($T$ is the set of cells with the highest level). case 3: $(s\neq0~\text{and}~\boldsymbol{g}_{0}^{0}(kn_1+i)=0)$ $t\Leftarrow~i,j\Leftarrow~(t-1),c_{(0,0)}^{kn_1+i}\Leftarrow~1$ (this cell has the lowest level); while ($s+1\leq~j\leq~t-1$) $c_{(0,0)}^{kn_1+j}\Leftarrow~c_{(0,0)}^{kn_1+j+1}+1,j\Leftarrow~j-1$ (this cell corresponds to the bit $1$). $s\Leftarrow0$. $i\Leftarrow~i+1$. $c_{(0,0)}^{kn_1+1}\Leftarrow~c_{(0,0)}^{kn_1+1+d_1}+1$ (push first cell in each block to the next-highest level); while $(l\in~T)\{$ $c_{(0,0)}^{kn_1+l}\Leftarrow\max\{c_{(0,0)}^{kn_1+1},c_{(0,0)}^{kn_1+l-1},c_{(0,0)}^{kn_1+l+1}\}+1$ (push this cell to the highest level); $T\Leftarrow~T\setminus\{l\}$$\}. i\Leftarrow3,T\Leftarrow\emptyset,s\Leftarrow0,k\Leftarrow~k+1 (initially, set some parameters for block k+1). until k=m-1. •  Algorithm 2 Transform the cell configuration \boldsymbol{c}_{{s_{i}}} into another cell configuration \boldsymbol{c}_{{s_{i+m}}} such that \boldsymbol{f}_{\boldsymbol{c}_{{s_{i}}}}=\boldsymbol{v}_{s_{i}} and \boldsymbol{f}_{{\boldsymbol{c}_{s_{i+m}}}}=\boldsymbol{v}_{s_{i+m}} Input: Current cell configuration \boldsymbol{c}_{{s_{i}}}, the initial value \boldsymbol{v}_{s_{i}}, new block value \boldsymbol{v}_{s_{i+m}}. Output: New cell configuration \boldsymbol{c}_{{s_{i+m}}}. k\Leftarrow~0,~T\Leftarrow\emptyset,~H_1\Leftarrow~\emptyset,~H_2\Leftarrow~\emptyset,~j\Leftarrow~3,~s\Leftarrow~0,~t\Leftarrow~0 \boldsymbol{c}_{{s_{i+m}}}(2)\Leftarrow\max\{\boldsymbol{c}_{{s_{i}}}(1),\boldsymbol{c}_{{s_{i}}}(3)\}+1 (push the 2nd cell to the highest level in \boldsymbol{c}_{{s_{i}}}). while (3\leq~j\leq~n_1)\{ case 1: (j\leq~d_1+2) \boldsymbol{c}_{{s_{i+m}}}(j)\Leftarrow\max\{\boldsymbol{c}_{{s_{i+m}}}(j-1),\boldsymbol{c}_{{s_{i}}}(j+1)\}+1 (push the jth cell in a run of 0s). case 2: (s=0~\text{and}~\boldsymbol{v}_{s_{i+m}}(j)=1) s\Leftarrow~j,T\Leftarrow~T\cup\{j\} (this cell is the highest in \boldsymbol{c}_{{s_{i+m}}}). if t\neq~0 then H_2\Leftarrow~H_2\cup\{(t,s)\} (H_2 is a set of runs of 0s). case 3: (s\neq0~\text{and}~\boldsymbol{v}_{s_{i+m}}(j)=0) t\Leftarrow~j (t represents the position of the lowest cell in \boldsymbol{v}_{s_{i+m}}). H_1\Leftarrow~H_1\cup\{(s,t)\} (H_1 is a set of runs of 1s). if \boldsymbol{v}_{s_{i}}(j-1)=0~\text{and}~\boldsymbol{v}_{s_{i}}(j)=1 then \boldsymbol{c}_{{s_{i+m}}}(j)\Leftarrow\boldsymbol{c}_{{s_{i}}}(j) (this is the highest cell in \boldsymbol{c}_{s_i}. Let the charge level of this cell remain unchanged); else \boldsymbol{c}_{{s_{i+m}}}(j)\Leftarrow\max\{\boldsymbol{c}_{{s_{i}}}(1),\boldsymbol{c}_{{s_{i}}}(j-1),\boldsymbol{c}_{{s_{i}}}(j+1)\}+1. s\Leftarrow0. j\Leftarrow~j+1. \boldsymbol{c}_{{s_{i+m}}}(1)\Leftarrow\boldsymbol{c}_{{s_{i+m}}}(1+d_1)+1 (push the first cell to next-highest level in the changed block); while big(A=(s_1,t_1)\in~H_1big) k\Leftarrow~(t_1-1) if (s_1+1\leq~k\leq~t_1-1) \boldsymbol{c}_{{s_{i+m}}}(k)\Leftarrow\max\{\boldsymbol{c}_{{s_{i}}}(k-1),\boldsymbol{c}_{{s_{i+m}}}(k+1)\}+1, k\Leftarrow~(k-1); H_1\Leftarrow~H_1\setminus\{A\}. while big(B=(t_1,s_1\big)\in~H_2) k\Leftarrow~(t_1+1); if (t_1+1\leq~k\leq~s_1-1) \boldsymbol{c}_{{s_{i+m}}}(k)\Leftarrow\max\{\boldsymbol{c}_{{s_{i+m}}}(k-1),\boldsymbol{c}_{{s_i}}(k+1)\}+1, k\Leftarrow~(k+1); H_2\Leftarrow~H_2\setminus\{B\}. while (l\in~T)\{ \boldsymbol{c}_{{s_{i+m}}}(l)\Leftarrow\max\{\boldsymbol{c}_{{s_{i+m}}}(1),\boldsymbol{c}_{{s_{i+m}}}(1-1),\boldsymbol{c}_{{s_{i+m}}}(1+1)\}+1(let the charge level of this cell be the highest in the changed block); T\Leftarrow~T\setminus\{l\}$$\}$.
• Table 2   Rates of our Gray code $G$ with respect to $d$
 $d$ $R=~\frac{{\log}_{2}N}{n}$ $13$ $\log_2\alpha_3=0.8543$ $15$ $\log_2\alpha_4=0.9468$ $17$ $\log_2\alpha_5=0.9752$ $d\rightarrow\infty$ $1$
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